A switch ON /OFF with the presence of a iron bar

I want to make switch which ON in the presence of an iorn bar .(when the iorn bar closely near the coil switch will be ON/OFF)

The iorn bar has two position .Postion 1 - the switch will ON
Position 2 the switch will OFF.

.I want to make it . Where i start , How to measure the coil voltage any simple idea for it ?

I would say that you should't concern on coil voltage but more on coil current, and measure the generated electromagnetic attraction force. There are freely available field solvers ( e.g FEMM42 ) with a lot of materials available at their libraries, so that you could try drawing this geometry there and make some experiments varyng either electric/geometric parameters to see which is the amount of current necessary to move the bar from that distance.

The schematic you have shown suggests a metal detector. It is a elementary version of metal detector where you sense the change in the inductance of the coil.

I think it is called inductive sensor, isn't it ?

Most inductive proximity sensors are using an oscillator circuit. They react more on change of coil Q than inductance. Bringing (mostly ferromagnetic) metals near to the sensor coil stops the oscillation by dampening the resonant circuit.

Sorry that I spoke in a hurry; you are right. It is kind of marginal oscillator or something like that, correct?

If the iron bar could be made magnetic a hall effect switch might simplify things.
(If - that is)

Actually am trying to make bike side stand detector
thanks